As light emitting devices such as light emitting diodes (LEDs) in which a single crystal substrate is used, light emitting devices in which various gallium nitride (GaN) layers are formed on sapphire (α-alumina single crystal) are known. For example, those having a structure formed by stacking on a sapphire substrate an n-type GaN layer, a multiple quantum well (MQW) layer in which a quantum well layer composed of an InGaN layer and a barrier layer composed of a GaN layer are alternately stacked, and a p-type GaN layer in this order are in mass production. Moreover, a multi-layer substrate suitable for such use is also proposed. For example, Patent Document 1 (JP2012-184144A) proposes a gallium nitride crystal multi-layer substrate including a sapphire base substrate and a gallium nitride crystal layer formed by crystal growth on the substrate.
When a GaN layer is formed on a sapphire substrate, dislocation is likely to occur because the lattice constant and the coefficient of thermal expansion of the GaN layer do not match with those of sapphire, which is a foreign substrate. Moreover, since sapphire is an insulating material, it is not possible to form an electrode on its surface, and, therefore, it is not possible to configure a light emitting device having a vertical structure that includes electrodes on the front and back of the device. Accordingly, LEDs in which various gallium nitride (GaN) layers are formed on a GaN single crystal have been attracting attention. Since a GaN single crystal substrate is made of the same type of material as a GaN layer, the lattice constants and the coefficients of thermal expansion are likely to match, and higher performance can be expected than the case where a sapphire substrate is used. For example, Patent Document 2 (JP2010-132556A) discloses a self-supporting n-type gallium nitride single crystal substrate having a thickness of 200 μm or greater.